As is known in the art, multipath induced signal fade can occur in different places at different frequencies. At a particular location, different frequencies can have different levels of attenuation. As is well known in the art, multipath fading can significantly degrade signal quality.
FIG. 1 shows fading effects of multipath for an aircraft at about 1500 feet above water communicating with an airborne object at an altitude of about 500-3000 feet. The datalink between the objects has multiple components including a direct path DP signal and reflected phase-shifted signals RPS that interfere constructively and destructively with the direct path signal.
The vertical axis represents the signal margin for a particular link, geometry, and frequency, which can be considered a measure of received signal strength in dB. The horizontal axis is the ground distance between the transmitter and the receiver antennas. It can be readily seen that the signal level decreases as the distance (range) increases. The scalloping of the signal is due to the destructive and constructive interference of the reflected signal with the direct signal due to multipath. When the difference between the direct path length and the reflected path length is a multiple of the wavelength, the signals constructively interact and a peak is produced. When the difference is an odd half multiple of the wavelength, the signals destructively interact and a null is produced.
There a variety of known multipath mitigation techniques. One such strategy increases the transmitter power so that even with multipath fading the received signal is still detectable. Another known technique includes estimating the amplitude and phase shift of the reflected signal component of the received signal for reconstructing the direct-path signal component using the estimates. A further known approach is set forth in IEEE 802.11x teaching wireless applications to employ multiple transmit and receive antennas, which is known as Multiple-Input, Multiple-Output (MIMO). As with previous techniques, it attempts to reconstruct the transmitted signal from a combination of the signals received from the multiple transmit antennas by the multiple receive antennas. This technique is relatively complex and requires additional assets including multiple transmitters and receivers.
The nulls of the fading occur when the indirect (reflected) path length differs from the direct path length by an odd half-multiple of the wavelength λ (½λ, 3/2λ, 5/2λ, . . . ). At a different frequency, and hence a different wavelength, the path length differences that give rise to the nulls will shift given the same geometry. In particular, they will occur at different direct path distances between the antennas, as shown in FIG. 2 which employs two different frequencies and plotting both results on one axis.